In general, a compression freezer includes a compressor, a condenser, an expansion valve and an evaporator. Within an enclosed system of the compression freezer, a liquid mixture of a refrigerant and lubricating oil is circulated. An example of a refrigerant conventionally used in many of such compression freezers is chlorofluorocarbon such as dichlorodifluoromethane (R12) or chlorodifluoromethane (R22). On the other hand, examples of the lubricating oil conventionally used therein are a variety of mineral oil and synthetic oil.
However, chlorofluorocarbon as mentioned above may bring about an environmental pollution such as destruction of the stratospheric ozone layer, a use of which has been more strictly regulated on a global scale in recent years. Thus, a hydrogen-containing chlorofluorocarbon compound such as hydrofluorocarbon or hydrochlorofluorocarbon is attracting more and more attentions as a new refrigerant. Such a hydrogen-containing chlorofluorocarbon compound, especially hydrofluorocarbon represented by R134a, is a preferable refrigerant for compression freezers not only because the compound does not destroy the ozone layer but also because the compound can be used in conventional freezers in place of conventional chlorofluorocarbon such as R12 without requiring configurations of the conventional freezers to be changed (e.g., see Patent Document 1).
On the other hand, since hydrofluorocarbon may also bring about an impact on the environment in terms of global warming, a so-called natural refrigerant such as carbon dioxide or ammonia is also attracting more and more attractions as an alternative refrigerant that is more suitable for environmental protection. Refrigerator oil using such a natural refrigerant has been also proposed (e.g., see Patent Document 2). In addition, a refrigerant having a specific polar structure in its molecule such as an unsaturated fluorohydrocarbon compound, a fluoroether compound, a fluoroalcohol compound or a fluoroketone compound has been found to have lower global warming potential (e.g., see Patent Documents 3 and 4).    Patent Document 1: JP-A-10-008078    Patent Document 2: JP-2000-96075    Patent Document 3: JP-T-2006-503961    Patent Document 4: JP-T-07-507342